[bloodhawk]

No, if you’re measuring from psu side to ground it should read +100 V from anode resistors and -100 V from cathode resistors, ei full B+ and B-. If you’re talking about the output stage.
.

Damn, then im way off. Any idea what to check for, to figure out what is causing the discrepancy? By the Anode/Cathode resistors i mean the 330R's .. Am i mistaken here or does this need to be measured at the 220R's?

All the connections / wires were resoldered to make sure things were in place.

Don’t understand what it is you’re measuring here? Pin 1 is one of the grids.

Hunh, i was under the impression Pin 1 was ground / NC ?

 

[SonicTrance]

Damn, then im way off. Any idea what to check for, to figure out what is causing the discrepancy? By the Anode/Cathode resistors i mean the 330R's .. Am i mistaken here or does this need to be measured at the 220R's?

All the connections / wires were resoldered to make sure things were in place.



Hunh, i was under the impression Pin 1 was ground / NC ?

You’re probably measuring from anode and cathode side of the resistors in which case the 79 V readings are normal (the readings at the cathode should be negative). 75 V are a little low as that would mean a 25 V drop across 330R, that’s 76 mA of bias.

Don’t know of any 220R resistors?

Yes, pin1 is grid. 6sn7/6080/6as7 has easy to learn pinout: grid, plate, cathode, grid, plate, cathode, heater, heater.

 

[baronbeehive]

So i think the bias issue is fixed. He has 2 bad 330R resistors whose values would go up by about 25Ohms as the amp started to heat up.

Im still now too confident though, usually with the 6080's it seems fine on when the amp is upside down on my workbench, but once i put it back together and set it up properly the bias deviates. Ill finish and test it again tomorrow.

For not the bias's match even after leaving the amp running for about 20 mins.

Ha, I had exactly this problem. Fortunately there is usually a simple solution. What that suggests to me is a bad connection somewhere which might look ok on the surface. The thing is when you put the parts back in their positions you have to bend wires and push caps back in place etc putting pressure on the joints. So what I would suggest is to check continuity on those areas you suspect on that problem circuit but this time wiggle the wires as you do the check, I suspect you might find continuity is disrupted on the problem areas. Those resistors going off is also a result of the problem, not the cause I suspect. Also I bet they are on that problem circuit.

 

[bloodhawk]

You’re probably measuring from anode and cathode side of the resistors in which case the 79 V readings are normal (the readings at the cathode should be negative). 75 V are a little low as that would mean a 25 V drop across 330R, that’s 76 mA of bias.

Don’t know of any 220R resistors?

Yes, pin1 is grid. 6sn7/6080/6as7 has easy to learn pinout: grid, plate, cathode, grid, plate, cathode, heater, heater.

Ah gotcha. So im guessing one of the PSU components isnt properly soldered. Any tips on what to check for the bias drop? Pretty much covered all the basic things like re tinning, making sure both sides of the via/pads are properly soldered along with checking each of the components out of the circuit.

I meant the 220k resistors, at the driver stage.

Really appreciate it!

Ha, I had exactly this problem. Fortunately there is usually a simple solution. What that suggests to me is a bad connection somewhere which might look ok on the surface. The thing is when you put the parts back in their positions you have to bend wires and push caps back in place etc putting pressure on the joints. So what I would suggest is to check continuity on those areas you suspect on that problem circuit but this time wiggle the wires as you do the check, I suspect you might find continuity is disrupted on the problem areas. Those resistors going off is also a result of the problem, not the cause I suspect. Also I bet they are on that problem circuit.

Will do ! Im literally this close to taking the whole thing apart and rewiring everything to my liking.. but only if this was my amp lol.

 

[baronbeehive]

Will do ! Im literally this close to taking the whole thing apart and rewiring everything to my liking.. but only if this was my amp lol.

Probably no need to go to those lengths lol.

Check every connection there, not just wires, all new parts, legs etc, give them a good wiggle because if they come off then they were not soldered properly anyway!It's possible that the solder looks good on the pad but the pad might have come unstuck with too much heat and just making feeble contact. Also people make the mistake of using too much solder which just hides the problem from view.

 

[bloodhawk]

Probably no need to go to those lengths lol.

Check every connection there, not just wires, all new parts, legs etc, give them a good wiggle because if they come off then they were not soldered properly anyway!It's possible that the solder looks good on the pad but the pad might have come unstuck with too much heat and just making feeble contact. Also people make the mistake of using too much solder which just hides the problem from view.

I resoldered all the new parts properly with continuity checks 2 days back. Only solder joints i havent checked are of the PSU side components. Guess i should do that as well.

 

[baronbeehive]

I resoldered all the new parts properly with continuity checks 2 days back. Only solder joints i havent checked are of the PSU side components. Guess i should do that as well.

If the bias is fine upside down but not when you put it back together, it's got to be what I said, that's the only thing that's changed.

 

[klnglim]

This has happened to me as well! Check the 100 V zener diodes, they’re most likely open. You’ve circled the B- zener but replace both. I used 3w zener s there followed by 5w resistors. Also check the voltage setting transistors. The one for B- is more than likely broke! Again, replace both. I can give you part number tomorrow when I’m at the computer as the transistors LD use are no longer used.

The transistor is L7812CV, I have just changed brand new, the original resistor show 2k2 ohm / 0.5 watts

 

[bloodhawk]

The transistor is L7812CV, I have just changed brand new, the original resistor show 2k2 ohm / 0.5 watts

You should check the Diode on top of it as well.

 

[klnglim]

This has happened to me as well! Check the 100 V zener diodes, they’re most likely open. You’ve circled the B- zener but replace both. I used 3w zener s there followed by 5w resistors. Also check the voltage setting transistors. The one for B- is more than likely broke! Again, replace both. I can give you part number tomorrow when I’m at the computer as the transistors LD use are no longer used.

ok thanks

 

[bloodhawk]

Annnd yeap the issue is with the tube sockets. Now the issue has moved to socket 2 xD

damn.

 

[klnglim]

You should check the Diode on top of it as well.

every where checked no problem except the place which resistor was burnt

 

[SonicTrance]

The transistor is L7812CV, I have just changed brand new, the original resistor show 2k2 ohm / 0.5 watts

That transistor is for the fan 12 V power supply. I'm talking about the transistors that are located between the power transformers, MJ15020 and MJ15021.
Check the wires that's coming from those transistors first as they're notorious for breaking. If connection is good check the transistors, the MJ15021 (B-) is probably broke. If so replace both with these:
MJ15024G (NPN) for B+
MJ15025G (PNP) for B-
Also, the zeners are more than likely open if there's smoke! You can use higher wattage resistors in that posistion for the 2k2 and 2k7. It's a week spot in this amp.

 

[bloodhawk]

That transistor is for the fan 12 V power supply. I'm talking about the transistors that are located between the power transformers, MJ15020 and MJ15021.
Check the wires that's coming from those transistors first as they're notorious for breaking. If connection is good check the transistors, the MJ15021 (B-) is probably broke. If so replace both with these:
MJ15024G (NPN) for B+
MJ15025G (PNP) for B-
Also, the zeners are more than likely open if there's smoke! You can use higher wattage resistors in that posistion for the 2k2 and 2k7. It's a week spot in this amp.

LOL talk about timing, one of the zeners blew on the amp i was working on because of the brittle ass wires (black one to be exact). (The top one to be specific, going to MJ15020 )


Any way to test if there was damage to the transistors?

Also do you have a link to where you got the 3w Zeners from ? Im ordering the transistors from mouser..

Once the new ones are here, ill just replace the garbage wire with some extended better quality wiring.

@baronbeehive I think the issue was party the heater wiring between the sockets as well. I swapped out the wires and the voltages were matching across both sides, at least before the diode went kablowie. xD

 

[SonicTrance]

Any way to test if there was damage to the transistors?

Here's how you test transistors on a DMM:

Step 1: (Base to Emitter)

Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see “OL” (Over Limit).

Step 2: (Base to Collector)

Keep the postitive lead on the BASE (B) and place the negative lead to the COLLECTOR (C).

For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see "OL" (Over Limit).

Step 3: (Emitter to Base)

Hook the positive lead from the multimeter to the to the EMITTER (E) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor.

For an good NPN transistor, you should see “OL” (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.

Step 4: (Collector to Base)

Hook the positive lead from the multimeter to the to the COLLECTOR (C) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor.

For an good NPN transistor, you should see “OL” (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.

Step 5: (Collector to Emitter)

Hook the postitive meter lead to the COLLECTOR (C) and the negative meter lead to the EMITTER (E) – A good NPN or PNP transistor will read "OL"/Over Limit on the meter. Swap the leads (Positive to Emitter and Negative to Collector) – Once again, a good NPN or PNP transistor should read “OL”.

If your bipolar transistor measures contrary to these steps, consider it to be bad.

Also do you have a link to where you got the 3w Zeners from ? Im ordering the transistors from mouser..

Just use any 2 or 3w 100 V zeners, nothing special. Would also recommend higher wattage resistors for the 2k2 and 2k7. Mills MRA5 is a tight fit in those holes. Here's mine.